Removal of volatile contaminants from the vadose zone of contaminated ground

ABSTRACT

Volatile contaminants are removed from the vadose zone of contaminated ground by pumping volatilized contaminants from the vadose zone using one or more vacuum extraction wells.

This is a continuation of application Ser. No. 567,972 filed Jan. 4,1984 now U.S. Pat. No. 4,590,760.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for removing volatile contaminantsfrom the vadose zone, also known as the zone of aeration or, theunsaturated zone, that is, the zone between the earth's surface and theunderground water table.

In the following description, the term "volatile liquid contaminant"shall mean the liquid phase of the contaminant and the vapor phase inequilibrium therewith in the subsurface media.

Contaminant of the substrate ground with potentially hazardous materialsis a common problem facing industry, the government and the generalpublic. Often, as a result of spills, leakage from storage facilities orsurface discharges, contaminants enter subsurface soil and/or rockformations. These contaminants eventually percolate into thegroundwater, thereby posing a health threat to drinking water supplies.Various methods for withdrawing and treating contaminated groundwaterare well known and widely practiced. However, methods fordecontaminating substrate soil and/or rock located in the vadose zone,above the water table, are limited. Generally, the contaminated soil inthe vadose zone is excavated and removed. Alternatively, thecontaminated soil can be left in place so that the contaminants can beleached out by percolating water and then the contaminated percolatingwater can be removed and recovered after it has reached the water table.The leaching of contaminants from the subsurface media is extremely slowfor many common contaminants because of their low solubility in waterand their absorption by soil and rock formations. As a result, remedialmeasures, directing at cleaning up and protecting the groundwater near acontamination site, must be performed for many years or even decades. Inaddition, the prior methods are ineffective for removing, from thevadose zone, contaminants having a low solubility in water and/or a highaffinity for soil, which class of contaminants includes most of thehazardous pollutants that affect groundwater. The present inventioninvolves removing volatile contaminants directly from the subsurfacemedia in the vadose zone above the water table.

2. Description of the Prior Art

Numerous techniques exist for removing gas that is formed naturally inlandfills and other constructed media. Circulation systems for leachingthe contaminants from contaminated media above the water table have beendesigned, but they invariably require that the leached contaminantsenter the water table or aquifer and be recovered by means of a waterremoval well and pump that draws water from the saturated zone oraquifer (see, for example, Forte et al, U.S. Pat. No. 4,167,973). Inaddition, it has been proposed to remove contaminant vapors fromcontaminated underground areas by means of a plurality of elongatedperforated collection elements installed in a large excavation (seeKnopik, U.S. Pat. No. 4,183,407). However, this system is limited toremoval of contaminants from depths for which excavation is practical,usually less than about 25 feet deep.

The present invention provides a method for removing volatilecontaminants from subsurface media, directly from any depth, within thevadose zone, without effecting hazardous and expensive excavation of alarge amount of contaminated soil or rock. The equipment used to performthe present invention can be conventional equipment, most of which canbe installed on the surface of the ground. Thus, the equipment need notbe specially manufactured or installed within the contaminated areabeneath the ground surface. The equipment can be installed with minimaldisruption of the subsurface media. This is especially important inindustrial environments in which excavation may be detrimental toindustrial operations or the integrity of industrial structures in thearea of contamination.

SUMMARY OF THE INVENTION

The present invention includes the steps of directly removing volatileliquid contaminant, which is precolating through the substrate media,from the subsurface media in the vadose zone above the water table andthen collecting, treating or otherwise disposing of the removedcontaminant. According to the invention, one or more wells are drilledinto the subsurface media in the contaminated area. The well isconstructed so that fluids in the vadose zone can flow into a well,whereas the liquid in the saturated zone below the water table cannotsubstantially flow into the well. The upper portion of the well isimpervious, whereas the lower portion of the well is perforate or opento the flow of fluid thereinto. The lower portion of the well issurrounded by a permeable material, such as gravel. The upper portion ofthe well is filled with a material of low vapor permeability whereby toinhibit leakage of air from the surface of the ground to the lowerperforate portion of the well. The upper end of the well is connected toa suitable vacuum source, for example, an exhaust fan, blower or vacuumpump, to create a relative pressure drop in the well, thus indicatingflow of volatile contaminants directly into the perforate lower portionof the well. Once the contaminants are in the well, they can be removedby means of the fan, blower or vacuum pump, and/or by means of anoptional submersible pump located at the bottom of the well. The exhaustof the fan, blower or vacuum pump, and/or the submersible pump, can beconnected to a suitable recovery, treatment and/or discharge system.

Thus, according to the invention, volatile contaminants are removed fromthe vadose zone of a contaminated underground area by a series of stepswhich comprises, firstly, establishing a borehole from the surface ofthe ground within the contaminated area by drilling or driving aborehole in any conventional manner. The borehole may extend partway tothe water table so that the bottom of the borehole is located in thevadose zone and may be spaced above the water table. However, theborehole and the hereinafter mentioned perforate lower portion of theconduit may extend into the saturated zone, in which case the perforatelower portion of the conduit will be effective to draw contaminant fromthe vadose zone and the vacuum in the conduit will be insufficient todraw a significant amount of water from the saturated zone into theconduit. However, if desired, a separate sampling device can be providedin the borehole for obtaining samples of the groundwater. A conduit orwell casing is inserted in and is radially inwardly spaced from theborehole so that a ring or annular zone is defined between the wall ofthe borehole and the conduit. The conduit has a perforate lower portionso that fluids can flow into the interior thereof, whereas the upperportion of the conduit is imperforate. The annular zone between theperforate lower portion of the conduit and the borehole wall is filledwith a loose, fluid-permeable, first, fill material which extendssubstantially up to the upper end of the perforate lower portion of theconduit. Thus, the pressure drop between the perforate lower portion ofthe conduit and the lower borehole wall portion opposed thereto will below, and the fluid-permeable fill material will easily permit fluid toflow into the perforate lower portion of the conduit when vacuum isapplied to the conduit. The annular zone above the perforate lowerportion of the conduit is packed with a second fill material of lowpermeability which is effective to impede flow of air from the groundsurface downwardly toward the lower portion of the borehole. Thus, theflow of air from above ground vertically downwardly into the conduit isminimized in order to increase the flow of the volatilized contaminantfrom the substrate media into the conduit. A vacuum is applied to theupper end of the conduit so as to draw vapor of the contaminant presentin the vadose zone into the lower portion of the conduit, thecontaminant vapor then flowing to the upper end of the conduit, fromwhence it can be fed to other equipment for treatment, recovery ordischarge. The perforate lower portion of the conduit is located in thevadose zone. When the perforate lower portion of the conduit extendsinto the saturated zone, no significant amount of groundwater is drawnfrom the saturated zone into the conduit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a vacuum extraction system according tothe invention.

FIG. 2 is a partial cross-sectional elevational view of a vacuumextraction well used in the system of FIG. 1.

FIG. 3 is a partial cross-sectional elevational view of a modificationof a vacuum extraction well.

DETAILED DESCRIPTION OF THE INVENTION

The invention is used to treat of substrate media in the vadose zone V(FIG. 1), which media is contaminated with one or more volatile liquidcontaminants. The volatile liquid contaminant may be percolatingdownwardly toward the water table, or it may be relatively stationary inthe vadose zone. For example, the contaminants can be aliphatic and/oraromatic hydrocarbons, halogenated hydrocarbons, or other volatileorganic compounds (VOCs). Such contaminants can be volatilized whensubjected to vacuum or a flow of air. The process of the invention ispreferably used for removal of contaminants having vapor pressures of atleast 20 mmHg at temperatures of 23° C. or less, and is most preferablyused for removal of contaminants having vapor pressures of 100 mmHg attemperatures of 23° C. or less. Contaminants having lower vaporpressures can be removed, but the rate of removing them is lower. Thepurpose of the invention is to volatilize and remove the contaminantsdirectly from the contaminated zone so as to minimize the amount ofcontaminant that enters the saturated zone, that is, the zone below thewater table WT (FIG. 1). The treatment is effected in situ withoutsubstantial excavation or other disturbance of the subsurface media. Theinvention is a cost effective procedure for removing contaminants and itis effective to shorten the time required to clean up the contamination.The invention is particularly good for removing water-insoluble orsparingly water-soluble contaminants, but it is also useful for removingwater-soluble contaminants.

The invention is particularly concerned with removing the contaminantfrom the vadose zone. However, the process according to the presentinvention can and normally will be used in combination with anadditional system or systems for reclaiming contaminated groundwaterbecause not all of the contaminant will be removed by the treatmentaccording to the invention. Thus, for example, one or more additionalpumps can be provided to remove groundwater from the aquifer, treat itand then recharge it into the aquifer after removing the contaminanttherefrom. The latter technique is well known in the art.

When contamination of the subsurface media has occurred and it isdesired to remove the contaminants therefrom, the region of thesubsurface media that is contaminated will be delineated by techniqueswhich are well known in the art. For, example, test borings can be madeat selected locations and at selected depths in and around thecontamination site to determine subsurface geologic conditions. Inaddition, chemical analyses of core samples taken during the testborings for determining geologic conditions are subjected to chemicalassay to delineate the extent, thickness, rate of spread and contents ofthe contamination in the unsaturated zone.

After the contamination area has been delineated as discussed above, thelocation or locations at which the installation of a vacuum extractionwell will be expected to be effective to remove contaminant from thesubsurface media will be determined, based on hydrogeologicconsiderations. A vacuum extraction well will then be installed at oneor more of those locations. The vacuum extraction well or wells may beinstalled in porous underground media, preferably permeable, fine-grainmaterials such as silts and sands, so that the well(s) will be effectiveto remove the contaminant that is present in the underground mediaaround the well.

If the subsurface media is relatively nonpermeable, such as clay, thevacuum extraction well can be operated so as to dry the surrounding clayso that it cracks and becomes more permeable whereby contaminants canflow into the well.

Referring to FIG. 1, the vacuum extraction well 10 is connected to asuitable vacuum source 11, such as an exhaust fan, blower or vacuumpump. The discharge from the vacuum source 11 is connected to anysuitable system for collecting, treating, or otherwise disposing of theremoved contaminants. In the illustrated embodiment of FIG. 1, the gasdischarged from the vacuum source 11 is flowed through a condenser 12wherein the water vapor and, possibly, some or all of the volatilizedcontaminant that is contained in the discharged gas may be condensed.The condensate and gas are then flowed into a storage tank 13. Thevolatile contaminant gas is then discharged into the ambient air throughthe stack 14. When discharge of the volatilized contaminant into theambient air is not feasible because of environmental constraints or thelike, the contaminant can be treated in other known ways, such as byadsorption, for example, by activated carbon, oxidation or otherchemical or biological treatment, to render same harmless. If thecontaminant is a useful substance, it can be collected for future use.During operation of the vacuum extraction well, the monitoring wells MWcan be used to sample the subsurface conditions around the vacuumextraction well and monitor the effectiveness of the treatment.

Referring to FIG. 2, the vacuum extraction well 10 is comprised of aborehole 21 which can be dug in the ground by any well-drilling methodsuitable for penetrating the subsurface media that is present at thecontamination site. A vertical well casing or conduit 22, which is ofsmaller diameter than the borehole 21, is installed substantiallycoaxially within the borehole so as to form an annular zone 23therebetween. The upper portion 22A of the pipe 22 is imperforate,whereas the lower portion 22B of the pipe is perforate so that fluid canflow therethrough from the subsurface media into the interior of thepipe. The lower portion 23B of the annular zone between the lowerportion 22B of the pipe 22 and the borehole wall is packed with a loose,fluid-permeable, first, fill material 24, such as gravel, sand or rock,so that the pressure crop between the lower perforated portion 22B ofthe pipe and the borehole wall will be small, whereby fluid present inthe region surrounding the lower portion of the borehole wall can easilyflow into the perforate lower portion 22B of the pipe 22. The upperportion 23A of the annular zone between the upper portion 22A of thepipe 22 and the borehole wall is filled with a low-permeabilitymaterial, such as soil, clay or cement. In the illustrated embodimentthere is shown a bentonite seal layer 26 made of bentonite ballsexpanded with water, which seal layer is located close to the upper endof the perforate lower portion 22B of the pipe 22. On top of thebentonite seal layer 26, there is provided a layer 27 of expansive groutwhich extends from the bentonite seal layer to the surface of theground. The purpose of the seal layer 26 and the layer 27 of lowpermeability material in the upper portion of the borehole is to impedeflow of air from the surface of the ground to the lower portion 22B ofthe borehole and to direct the vacuum to zones of higher contamination.

When the vacuum source 11 is turned on, if the subsurface media is aporous permeable material the absolute pressure in the vicinity of theperforate lower portion 22B of the pipe 22 is reduced and, thereby, flowof air, contaminant vapor and liquid is induced through the contaminatedmedia and into the perforate lower portion of the pipe 22. The rate ofvolatilization of the contaminant is thereby increased and theequilibrium concentration of the contaminant in the vapor phase is alsoincreased. Thus, the contaminant is drawn into the perforate lowerportion of the pipe 22 and is brought to the upper end of the pipe 22whereat it can be treated, collected or otherwise disposed of.

If the subsurface media is relatively nonporous, impermeable,water-bearing clay, it is necessary to apply a sufficiently high vacuumto dry out the clay around the borehole to cause it to crack and therebybecome more permeable so that the contaminant can flow into the pipe 22.Thus, when the vacuum source is turned on, it is preferable to establisha pressure of less than 10 mmHg to dry the wall of the borehole. Oncethe borehole wall has been dried, the operation can proceed in the samemanner as described above.

The source of the air that flows into the perforate lower portion 22B ofthe pipe 22 can be the air that is naturally present in the substratemedia or that infiltrates into the surface media from the ambient air atthe ground surface. In addition, air for volatilizing the contaminantcan be artificially provided by drilling air supply wells or makingexcavations in the area surrounding the vacuum extraction well 10 inorder to promote the flow of fresh air into the subsurface media. Therecharging of fresh air into the subsurface media can be passive oractive. In passive recharging, an open well or excavation is provided toallow fresh air to enter in response to a pressure gradient. In activerecharging, fresh air is pumped under superatmosphere pressure into thesubsurface media near the vacuum extraction well 10.

In many installations, liquid water, aqueous contaminant and liquidcontaminant will accumulate in the perforate lower portion 22B of thepipe 22 because some of the water that is percolating through thesubsurface media toward the water table will be drawn into the pipe 22by the vacuum. In order to remove this water and any other liquid thatmay become present in the perforate lower portion 22B of the pipe 22, asubmersible pump 28 can be installed close to the bottom of the pipe 22.When the submersible pump 28 is in operation, water and other liquids,including possibly liquid contaminants, are pumped out of the well 10 inthe liquid phase. The liquid can be treated in order to remove thecontaminant, for example, they can be flowed directly into the tank 13.The pump 28 can be turned on and turned off in response to the liquidlevel in the pipe 22, as determined by liquid level probes (not shown)in a conventional manner.

The objective of the vacuum extractions of the contaminant from thevadose zone is to minimize the amount of contaminant that enters theaquifer. The most effective zone of accomplishing vacuum extraction ofthe contaminant is the zone directly above and close to the water tablebecause in that zone, the width of the zone around the vacuum extractionwell in which a significant vacuum is present is maximized. Also, thecontaminant may tend to accumulate at the surface of the water tablewhen the contaminant is of low water solubility. Thus, it will bepreferred to locate the bottom of the perforate lower portion 22B of thepipe 22 close to, but spaced vertically above, the water table WT in thesubsurface media being treated. The distance between the bottom of theperforate lower portion 22B and the water table will be determined basedon the zone of influence of the vacuum extraction well and the locationof the contaminated area.

The vertical height of the upper portion 23A of the annular zone that ispacked with the fill material of low permeability, will be chosen tominimize as much as possible the infiltration of air from the groundsurface into the lower portion of the borehole.

In another modification of the invention, as illustrated in FIG.3, theborehole may be drilled into the saturated zone and a pump may beinstalled to pump water therefrom through a first passageway, whilesimultaneously a perforate zone may be provided in the boreholecommunicating with the vadose zone to draw vapor therefrom through asecond passageway separate from the first passageway. Thus, the samewell can be used as a well for monitoring and removing ground water, andfor separately removing volatilized contaminant.

The method according to the invention is effective to remove substantialqualities of volatile contaminant that is present in the subsurfacemedia above the water table whereby to minimize the amount of thecontaminant that enters the groundwater. The nature and amount of thecontaminant that is present in the subsurface media, the vapor pressureof the contaminant, the characteristics of the subsurface media and thenumber and sizes of the vacuum extraction wells that are used, arefactors that affect the total treatment time and overall effectivenessof the treatment. Although it may be necessary to carry out thetreatment for a long period of time, it is likely that the treatmenttime will be shorter than the time that would be required if the onlytreatment performed was to pump water from the aquifer and reclaim it.The invention will be particularly advantageous and effective at theearly stages of treating the contamination site because it can achieverelatively rapid removal of relatively large quantities of thecontaminant. It may also be effective to reduce the spreading of thecontaminant in the subsurface media. Thus, if the nature of thecontamination requires the use of one or more removal wells for removingcontaminated water from the aquifer, separating the contaminant from thewater and recharging the decontaminated water into the aquifer, theamount of actually contaminated water that must be pumped out andtreated can be reduced so as to make it possible to use a smaller pump,a lesser number of pumps and/or a shorter treatment time period forremoving contaminated water from the aquifer.

Although a particular preferred embodiment of the invention has beendisclosed in detail for illustrative purposes, it will be recognizedthat variations or modification of the disclosed apparatus, includingthe rearrangement of parts, lie within the scope of the presentinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A process for removing anot-naturally-occurring, volatile liquid contaminant from a contaminatedaraa of the earth, wherein the contaminated area of the earth has asubsurface water table and a subsurface vadose zone above the watertable, the contaminant being present in the vadose zone, which comprisesthe steps of:establishing a borehole extending downwardly from thesurface of the ground within the contaminated area; placing a firstconduit within said borehole so that a ring-shaped zone is formedbetween the wall of said borehole and said conduit, said conduit havinga perforate lower portion located in the vadose zone and spacedvertically above the water table so that fluids can flow from the vadosezone into the interior of said conduit, said perforate lower portionhaving an upper end and a lower end, said conduit having an imperforateupper portion above the upper end of said perforate lowerportion.[.,.]..Iadd.; .Iaddend. filling fluid-permeable first fillmaterial into a lower portion of said ring-shaped zone around saidperforate lower portion of said conduit and which extends to the upperend of said perforate lower portion so that said first fill materialwill permit fluid readily to flow into said perforate lower portion ofsaid conduit; filling a second fill material of low fluid permeabilityinto an upper portion of said ring-shaped zone around said imperforateupper portion of said conduit, said second fill material being effectiveto impede flow of air from the surface of the ground to the lowerportion of the borehole; applying a vacuum to the upper end of saidconduit effective to induce flow of air and a gas containing vapor ofsaid contaminant present in said vadose zone into the lower portion ofsaid conduit and thence to the upper end of said conduit, the vacuumbeing effective to cause vaporization of a substantial quantity of saidcontaminant within the vadose zone around the borehole so that saidcontaminant is removed from the ground in vapor form.
 2. A processaccording to claim 1 including the additional step of condensing watervapor and condensible contaminant present in said gas and therebyseparating the condensed liquids from the remainder of said gas.
 3. Aprocess for removing a not-naturally-occurring, volatile liquidcontaminant from a contaminated area of the earth, wherein thecontaminated area of the earth has a subsurface water table and asubsurface vadose zone above the water table, the contaminant beingpresent in the vadose zone, which comprises the steps of:delineating theextent, thickness and contents of the contaminant and also determiningits direction and rate of spreading in the vadose zone; establishing atleast one borehole extending downwardly from the surface of the groundwithin the .[.contaiminated.]. .Iadd.contaminated .Iaddend.area; placinga first conduit within said borehole so that a ring-shaped zone isformed between the wall of said borehole and said conduit, said conduithaving a perforate lower portion located in the vadose zone and spacedvertically above the water table so that fluids can flow from the vadosezone into the interior of said conduit, said perforate lower portionhaving an upper end and a lower end, said conduit having an imperforateupper portion above the upper end of said perforate lowerportion.[.,.]..Iadd.;.Iaddend. filling fluid-permeable first fillmaterial into a lower portion of said ring-shaped zone around saidperforate lower portion of said conduit and which extends to the upperend of said perforate lower portion so that said first fill materialwill permit fluid readily to flow into said perforate lower portion ofsaid conduit; filling a second fill material of low fluid permeabilityinto an upper portion of said ring-shaped zone around said imperforateupper portion of said conduit, said second fill material being effectiveto impede flow of air from the surface of the ground to the lowerportion of the borehole; applying a vacuum to the upper end of saidconduit effective to induce only flow of air and a gas containing vaporof said contaminant present in said vadose zone into the lower portionof said conduit and thence to the upper end of said conduit, the vacuumbeing effective to cause vaporization of a substantial quantity of saidcontaminant within the vadose zone around the borehole so that saidcontaminant is removed from the ground in vapor form.
 4. A process asclaimed in claim 3 including the steps of pumping contaminated waterfrom below the water table; treating the contaminated water to removethe contaminant therefrom; and then recharging the decontaminated waterto the ground.
 5. A process as claimed in claim 3 including the step oftreating the volatilized contaminant discharged from the conduit bymechanical, chemical or biological treatment to render it harmless.
 6. Aprocess as claimed in claim 3 in which the contaminated area comprisesclayey subsurface media and in which the step of applying said vacuumcomprises first establishing a pressure of less than about 10 mmHg inthe conduit to dry out the clay around the conduit and cause it to crackand become more permeable, and then applying said vacuum to volatilizethe contaminant, draw it into the well and then discharge it aboveground.
 7. A process as claimed in claim 3 in which said perforate lowerportion of said .[.well casing.]. .Iadd.conduit .Iaddend.is locateddirectly above and close to the water table.
 8. A process fordecontaminating an underground vadose zone which is located above thewater table and is contaminated with a volatile liquid contaminant whichis percolating downwardly through said vadose zone, whichcomprises:establishing a vacuum extraction well comprising a well casinghaving a perforate lower portion located in the contaminated vadose zoneand above the water table so that fluids can flow from said contaminatedvadose zone into said perforate lower portion of said well casing, thearea around said vacuum extraction well casing above said perforatelower portion being substantially sealed to impede flow of air aroundsaid well casing from the surface of the ground to said perforate lowerportion of said well casing; applying a vacuum through said vacuumextraction well to said perforate lower portion of said well casing,said vacuum being effective to draw air through said contaminated vadosezone and to volatilize liquid contaminant that is present in saidcontaminated vadose zone above the water table and surrounding saidperforate lower portion of said well .[.caisng.]. .Iadd.casing.Iaddend.the vapor containing the volatilized liquid contaminant beingdrawn into said well casing and thence being transported to a locationabove ground.